1. Field of the Invention
The subject invention pertains to the field of a.c. potentiometers and more particularly to a.c. potentiometers wherein the ratio of the a.c. output signal to the a.c. input signal is directly proportional to the ratio of two d.c. signals.
2. Description of the Prior Art
In many applications, electrical data is supplied from reference instruments as a d.c. signal referenced to an independent or dependent d.c. source while, in many instances, an independant user of the reference data requires an a.c. signal reference to an independent a.c. source. For example, an air data computer may provide an altitude error output signal in the form of a d.c. signal referenced to an aircraft d.c. bus to a user equipment, such as an autopilot or flight director, that requires the altitude error signal as an independent a.c. signal. Prior art provided a motor generator in the air data computer or in the autopilot which was responsive to the d.c. error signal and provided an analog type output in the form of a mechanical shaft angle displacement. The shaft drove the wiper arm of a potentiometer, to which a reference a.c. voltage was applied from by the user equipment. The output voltage produced at the wiper arm was related to the reference voltage in accordance with the ratio of that portion of the resistance between the wiper arm and one end of the resistive element and the total resistance of the resistive element. This technology enables the electrical circuits in the user system to be completely isolated from the electrical circuits contained in the sensor or generator system. However, it is not only expensive, relatively unreliable and bulky, but also is inaccurate due to the hysteresis caused by the inherent backlash in the mechanism. Previous attempts to replace the mechanical elements with electronic elements have resulted in systems such as those that employ electronic IC multipliers, which do not provide sufficient accuracy, and simple modulators, which provide a.c. output signals that are only proportional to the d.c. data signal and not to the d.c. data signal and the a.c. reference or excitation signal.
The present invention provides an isolated a.c. output signal that closely resembles the output signal of a potentiometer, which is proportional in amplitude to both an a.c. excitation signal and a d.c. data signal.